Luo Qi, Li Jun, Cenkci Belgin, Kretzner Leo
Department of Cellular and Molecular Biology, Division of Basic Biomedical Sciences, University of South Dakota School of Medicine, Vermillion, SD 57069, USA.
Oncogene. 2004 Feb 5;23(5):1088-97. doi: 10.1038/sj.onc.1207225.
Myc proteins are transcriptional activators, but also repress transcription through initiator (Inr) elements. Repression requires the conserved Myc Box II, but the cis-acting element(s) required for c-myc autorepression have eluded definition. Since the gene has a candidate Inr at the P2 promoter, we tested whether Myc autorepression operates through the Inr/BoxII mechanism. Overexpression of c-Myc but not a Box II deletion mutation represses both c-myc P2 reporter genes and endogenous c-myc, as does Mxi1 expression. Only 45 nucleotides surrounding the P2 start suffice to mediate autorepression, but Myc and Mxi1 also downregulate P2 Inr mutations, suggesting other core promoter sequence requirements for autorepression. We tested the importance of conserved E2F sites, based on known Myc interaction with the pRb-related p107 and on the transrepressive effects of Rb family proteins. Myc, Mxi1, and p107 repress c-myc somewhat less well in the absence of E2F binding sites, while an E2F+Inr double mutation is not repressed at all by these gene products. Further, Myc repression at the c-myc P2 core promoter is augmented by p107, but not by pRb or p130, nor by p107 lacking the conserved pocket domain. Our data suggest that Myc autorepression requires both the c-myc Inr and E2F sites in cis, as well as p107 in trans. Consistent with this, we found that retrovirally transduced c-Myc cannot downregulate endogenous c-myc in p107-null fibroblasts, and show evidence that both Myc and p107 are present in a complex assembled at the c-myc P2 core promoter.
Myc蛋白是转录激活因子，但也通过起始子（Inr）元件抑制转录。抑制作用需要保守的Myc Box II，但c-myc自身抑制所需的顺式作用元件尚未明确。由于该基因在P2启动子处有一个候选Inr，我们测试了Myc自身抑制是否通过Inr/BoxII机制发挥作用。c-Myc的过表达而非Box II缺失突变可抑制c-myc P2报告基因和内源性c-myc，Mxi1的表达也有此作用。P2起始位点周围仅45个核苷酸就足以介导自身抑制，但Myc和Mxi1也会下调P2 Inr突变，这表明自身抑制还需要其他核心启动子序列。基于已知的Myc与pRb相关的p107的相互作用以及Rb家族蛋白的反式抑制作用，我们测试了保守的E2F位点的重要性。在没有E2F结合位点的情况下，Myc、Mxi1和p107对c-myc的抑制作用略有减弱，而E2F+Inr双突变则完全不受这些基因产物的抑制。此外，p107可增强c-myc P2核心启动子处的Myc抑制作用，但pRb或p130则不能，缺乏保守口袋结构域的p107也不能。我们的数据表明，Myc自身抑制在顺式作用中需要c-myc Inr和E2F位点，在反式作用中需要p107。与此一致的是，我们发现逆转录病毒转导的c-Myc不能下调p107缺失的成纤维细胞中的内源性c-myc，并证明Myc和p107都存在于在c-myc P2核心启动子处组装的复合物中。
